Over the years swimmers have been attempting to improve moving through water. Originally boards were attached to one's hands or feet, and have been used for over a hundred years with literally hundreds of variations. However, their hydrodynamic efficiency has been relatively poor in view of the difficulty of dealing with two human legs and allowing the fins to pass one another without collision. Current swim fins have evolved to an elongated flexible propulsion surface where their proliferation is mainly attributed to the ease of manufacture. All of these swim fins have suffered from problems of a very low aspect ratio and poor angle of attack. Other types of efficient swim assistance aids exist but have complexity and manufacturing costs that keep these aids from being used.
Earlier swim fin designs have problems with the aspect ratio and induced drag such as documented in U.S. Pat. No. 5,746,631 to McCarthy (1998) which is incorporated by reference. A substantial amount of induced drag is created by the transverse travel and vortex of fluid near the lateral edges of a lifting body (or foil) when that foil travels through a fluid. This induced drag reduces the effectiveness of the remainder of the foil. It has been established that a greater distance between the lateral edges improves the effective lift to drag ratio of the foil. The aspect ratio measures the separation of the lateral edges to the chord of the foil and is an indicator of the efficiency of the foil.
Most modern fins have an aspect ratio between 0.3 and 0.5. It is well known that a higher aspect ratio produces higher hydrodynamic efficiency. Many examples of this can be found in nature. Fish tails have widely varying aspect ratios. The fast swimming amberjack has a tail fin with an aspect ratio of about 8 while the much slower swimming grouper has an aspect ratio of about 1.5. Whales and dolphins have aspect ratios in the 5 to 6 range.
The angle of attack of a foil also affects the lift to drag characteristics of the foil. The angle of attack is the relative angle that exists between the actual alignment of the oncoming flow and the lengthwise alignment of the foil (or chord line). When this angle is small, the foil is at a low angle of attack. When this angle is high, the foil is at a high angle of attack. As the angle of attack increases, the flow collides with the foil's high pressure surface (also called the attacking surface) at a greater angle. This increases fluid pressure against this surface. While this occurs, the fluid curves around the opposite surface, and therefore must flow over an increased distance. As a result, the fluid flows at an increased rate over this opposite surface in order to keep pace with the fluid flowing across the attacking surface. This lowers the fluid pressure over this opposite surface while the fluid pressure along the attacking surface is comparatively higher. The pressure differential results in lift, or force causing it move in the direction of the low pressure.
A foil has an optimum angle of attack where the lift to drag ratio is the highest. When the foil is at a lower angle of attack than the optimum the lift is reduced with relatively little change in drag. When the foil is a higher angle of attack the drag increases substantially while the lift increases at a lesser rate. The increased drag is due to flow separation and the creation of turbulence on the low pressure side of the foil which is known as stalling. A typical optimum angle of attack for a foil is between 4 to 10 degrees. The angle of attack for most swim fins is 90 degrees which is in the stalled range and results in the swimmer having undue ankle stress and leg fatigue.
U.S. Pat. No. 2,729,832 to Schmitz described an improvement in efficiency by aligning the propulsion surface with the travel direction rather than the sole of the foot, but did not resolve low aspect ratio and extreme angle of attack having inefficiency created by vortices.
U.S. Pat. No. 107,376 to Hunter described a method for propelling ships using an oscillating rudder with multiple rubber propulsion vanes for ships.
U.S. Pat. No. 3,122,759 to Gongwer described improving performance with a very efficient high aspect ratio hydrofoil of about three feet laterally. The device resulted in propelling the swimmer in a straight line in open water but its size and complexity made it impractical for common sporting use.
U.S. Pat. No. 4,767,368 to Ciccotelli had a simpler high aspect ratio swim fin which was impractical for maneuvering in restricted areas and can cause significant stress on the swimmer's ankles due to the long lever arm from the ankle to the lifting vane. Generally, these high aspect ratio swim fins had protrusions which could snag underwater obstacles.
U.S. Pat. No. 4,781,637 to Caires described a high aspect ratio swim fin that required the swimmer to place both feet into the foot pocket requiring the swimmer to simultaneously kick both feet which was only useful in open water free of obstacles.
U.S. Pat. No. 4,178,128 Gongwer described a multi-vane hydrofoil shape swim fin to improve efficiency but required springs, hinges, and thin rods resulting in being mechanically complex, difficult to manufacture, prone to snagging underwater flora, and subject to abrasive wear from suspended grit.
U.S. Pat. No. 4,944,703 to Mosier showed a swim fin having multiple articulating hydrofoil vanes. However, the composite construction of internal rigid parts molded into less rigid parts resulted in expensive manufacturing costs. The 19 shown discrete parts in the figures indicate either manual assembly or a complex automated assembly line would be necessary resulting in expensive manufacturing costs. An implementation relied on pin and socket hinges and rubber inserts to control the articulation of the vanes which is subject to clogging and jamming by sand and other waterborne debris. The rigid side support beams would cause undue stress on the swimmer's ankles. The small gaps between the vanes and side beams and at the hinges are prone to trap stringy aquatic fauna and other stringy debris which may be encountered in the water creating a potential entrapment problem and a serious safety hazard.
An alternate embodiment in FIG. 6 of the Mosier '703 patent shows a resilient (rubber) hinge as the method of providing a rotational axis and self aligning of the vanes. However, this configuration would not work if physically constructed. Given the axial rotation desired of about 90 degrees as shown in FIG. 7 the axial length of the resilient hinge is too short to allow the rotation without overstressing the material and causing a shear failure. If the axial length were increased the narrow diameter would permit the vane to move out of alignment with the axis. The resilient hinge geometry has very high stress areas created at the interfaces between the softer and harder materials further increasing the likelihood of hinge failure. During operation, there would be no hard limit to the rotation of the vane. As more power is applied to the stroke, the vanes would rotate further reducing the effective lift of the vanes. Manufacturing would be difficult since five separately molded pieces would have to be hand placed in a second mold for the over molding process. Any manufacturing process which requires human interaction necessarily increases the cost.
U.S. Pat. No. 5,536,190 to Althen shows a propulsion method with an appropriate angle of attack using vane rotation limiters, high aspect ratio and plural vanes, but is hindered by many hinges and small parts which cause expensive manufacturing costs with the product prone to breakage and wear from captured grit. This device is impractical in aquatic environments since its parts can become entangled with flora.
U.S. Pat. No. 5,746,631 to McCarthy shows a fin with a longitudinal gap effectively creating a fin with propulsion surfaces which swing sideways during the power stroke. The apparatus reduces ankle stress but makes it difficult to attain higher rates of speed.
U.S. Pat. No. 3,084,355 to Ciccotelli uses narrow vanes which rotate along a transverse axis and are mounted parallel to each other in a direction that is perpendicular to the direction of swimming with vanes that are not hydrodynamically streamlined to generate lift, and no system is used to control tip vortices. The vanes are arranged so they only provide resistance to the kick during a small portion of the kicking stroke. When they are providing resistance they are effectively joined resulting in a lower aspect ratio vane than they are individually. Only two of the four vanes are functioning at any one time which leads to a cumbersome arrangement reducing the ability of the swimmer to control his attitude in a non-mobile condition. The device is overly complex and contains many small parts which are prone to corrosion, grit accumulation and snags.
U.S. Pat. No. 4,209,866 to Loeffler describes a thin pivotally mounted vane with reversibly effective streamline camber, but has a low aspect ratio which is known to have lower efficiency than higher aspect ratio vanes. The device was of complex construction with many wear points increasing the manufacturing and maintenance costs.
U.S. Pat. No. 5,330,377 to Kernek shows a swim fin with multiple connected surfaces creating channelized flow between them. The large surface area of the propulsion surfaces created sufficient viscous drag to cancel any gained benefit and the complex molding indicate a high fabrication cost.
U.S. Pat. No. 6,290,561 shows a swim fin with a propulsion surface supported by an elastic band and external beams. The elastic support restricts the maximum deflection of the propulsive surface but does nothing to control flow along the lateral surface edges. The edge vortices would create increased induced drag between the propulsion surface and support beams causing a reduction in efficiency compared to conventional swim fins.
U.S. Pat. App. 2009/0088036 to Garofalo shows a swim fin with restrained trailing edge and loose sides. The lack of a gap between the foot pocket and the vane eliminates the small benefit of its improved angle of attack. It includes “deformable folding side pockets which will be able not only to ensure a good “channel effect” but also to operate as deformation limiters.” The long longitudinal length of the side pockets is sufficiently long that the vortex limiting capability is reduced. The volume of channelized flow is large enough that it creates a cushion effectively acting as a new hydraulic surface which forces the free flow to move laterally and create new edge vortices.
U.S. Pat. No. 5,634,613 to McCarthy shows tip vortex canceling devices and U.S. Pat. No. 3,411,165 to Murdoch and U.S. Pat. No. 4,738,645 to Garofalo use pleats with composite construction to increase local deflection of the propulsion surface. However, these swim fins have low aspect ratios with the problems previously described.
U.S. Pat. No. 4,981,454 to Klein and U.S. Pat. No. 7,462,085 to Moyal show swim fins with a hinge on the foot pocket allowing the propulsion surface to rotate upward against the swimmer's shin to facilitate simplified walking while wearing the device. However, these devices are limited in their efficiency since they use conventional flat low aspect ratio propulsion surfaces subject to all the problems previously described.
Hinges using rubber like substances to provide torsional resistance are shown in U.S. Pat. No. 2,987,332 to Bonmartini and U.S. Pat. No. 4,097,958 to Van Dell that use composites of rubber and metal. The metal provides support for the hinge while the rubber provides the torsional resistance. However, the metal parts are not practical in a salt water environment, and their geometry requires a relatively large area for the installation of the hinge which would reduce the area allotted for the attached vanes.
The ScubaPro Nova SeaWing swim fin uses a flexible support beam combined with a very flexible root section of the support beam which allows the entire support beam to rotate in excess of 30 degrees. Additional flexing of the support beam allows a total flex in excess of 40 degrees which is what is considered the optimum angle of attack. The SeaWing, while innovative, still suffers from adverse propulsion surface curvature, a low aspect ratio, the lack of a hydrodynamic lifting surface, and insufficient control of tip vortices.
Thus, the need exists for solutions to the above problems with the prior art.